setuid (1)

setuid username|uid   command [ args ]

Setuid changes user id, then executes the specified command. Unlike some versions of su(1), this program doesn't ever ask for a password when executed with effective uid=root. This program doesn't change the environment; it only changes the uid and then uses execvp() to find the command in the path, and execute it. (If the command is a script, execvp() passes the command name to /bin/sh for processing.)

For example,

setuid some_user $SHELL

can be used to start a shell running as another user.

Setuid is useful inside scripts that are being run by a setuid-root user – such as a script invoked with super, so that the script can execute some commands using the uid of the original user, instead of root. This allows unsafe commands (such as editors and pagers) to be used in a non-root mode inside a super script. For example, an operator with permission to modify a certain protected_file could use a super command that simply does:

cp protected_file temp_file
setuid $ORIG_USER ${EDITOR:-/bin/vi} temp_file
cp temp_file protected_file

(Note: don't use this example directly. If the temp_file can somehow be replaced by another user, as might be the case if it's kept in a temporary directory, there will be a race condition in the time between editing the temporary file and copying it back to the protected file.)

Will Deich